The instant invention is in the field of optical correlators for use in the non-contact measurement of velocity and/or direction of motion and/or distance of an object. Such correlators generally comprise an imaging system having at least one grating structure mounted in the vicinity of the image plane of said system, at least one photoelectric receiver system arranged after the grating structure in the direction of the light for converting the modulated light fluxes resulting from the grating structure into electrical signals.
To use such an optical correlator, it is in most cases merely necessary to produce an image of the grating structure in a directional axis of the grating structure used in the analysis, preferably perpendicular to the direction of the lines of the grating. The quality of the image of the imaging system will be determined by the lattice constant of the analyzing grating structure, which acts as a spatial frequency filter.
It has been found that object structures with images smaller than one-half of the constant of the grating structures produced in the grating plane by the imaging system, are noticed in the signal generated by the electrical receiver system in the form of interfering harmonics, and that in the case of relatively coarse gratings the power of resolution of the imaging system does not provide adequate attenuation of these harmonics.